Crude oils and middle distillates such as gas oil, diesel oil or heating oil, obtained by distillation of crude oils, contain, depending on the origin of the crude oils, different amounts of n-paraffins, which crystallize out as lamellar crystals when the temperature is lowered and in some cases agglomerate with inclusion of oil. This results in a deterioration in the flow properties of these oils or distillates, giving rise to problems, for example in the recovery, transport, storage and/or use of the mineral oils and mineral oil distillates. In the case of mineral oils, this crystallization phenomenon can lead to deposits on the pipe walls during transport through pipelines, especially in the winter, and in individual cases, for example when the pipeline is shut down, even to complete blockage thereof. The precipitation of paraffins can also cause difficulties in storage and further processing of the mineral oils. Thus, it may be necessary in winter to store the mineral oils in heated tanks. In the case of mineral oil distillates, blockage of the filters in diesel engines and furnaces may occur owing to the crystallization, with the result that reliable metering of the fuels is prevented and complete interruption of the fuel or heating medium feed may occur.
In addition to the traditional methods for eliminating the paraffins which have crystallized out (thermally, mechanically or by means of solvents), which relate only to the removal of the precipitates already formed, recent years have seen the development of chemical additives (so-called flow improvers or paraffin inhibitors) which physically interact with the precipitating paraffin crystals and thus modify their shape, size and adhesion properties. The additives act as additional crystal seeds and partially crystallize out with the paraffins, resulting in a larger number of smaller paraffin crystals with modified crystal shapes. A part of the action of the additives is also explained by dispersing of the paraffin crystals. Modified paraffin crystals have less tendency to agglomerate, so that the oils into which additives have been introduced can be pumped or processed even at temperatures which are often more than 20° lower than in the case of oils not containing additives.
The flow and low-temperature behavior of mineral oils and mineral oil distillates is described by stating the pour point (determined according to ISO 3016) and the cold filter plugging point (CFPP; determined according to EN 116). Both characteristics are measured in °C.
Typical flow improvers for crude oil and middle oil distillates are copolymers of ethylene with carboxylic esters of vinyl alcohol. Thus, according to DE-A-11 47 799, oil-soluble copolymers of ethylene and vinyl acetate having a molecular weight between about 1,000 and 3,000 are added to mineral oil distillate fuels having a boiling point between about 120 and 400° C. Copolymers which contain from about 60 to 99% by weight of ethylene and from about 1 to 40% by weight of vinyl acetate are preferred. They are particularly effective if they were prepared by free radical polymerization in an inert solvent at temperatures of from about 70 to 130° C. and pressures of from 35 to 2,100 atm (gauge pressure) (DE-A-19 14 756).
Other polymers used as flow improvers contain, in addition to ethylene and vinyl acetate, for example 1-hexene (cf. EP-A-0 184 083), diisobutylene (cf. EP-A-0 203 554) or an isoolefin of the formula in which R and R′ are identical or different and are hydrogen or C1-C4-alkyl radicals (EP-A-0 099 646). Copolymers of ethylene, alkenecarboxylic esters and/or vinyl esters and vinyl ketone are also used as pour point depressants and for improving the flow behavior of crude oils and middle distillates of crude oils (EP-A-0 111 888).
In addition, copolymers based on α,β-unsaturated compounds and maleic anhydride are also used as flow improvers. DE-196 45 603 describes copolymers of from 60 to 99 mol % of structural units derived from ethylene and from 1 to 40 mol % of structural units which are derived from maleic acid, its anhydride or its imides.
DE-1 162 630 discloses copolymers of ethylene and vinyl esters of straight-chain fatty acids having 4 to 18 carbon atoms as a pour point-depressing additive for distillate fuels having a medium boiling point, such as heating oil or diesel oil.
EP-A-0 217 602 discloses ethylene copolymers with vinyl esters carrying C1- to C18-alkyl radicals as flow improvers for mineral oil distillates having boiling ranges (90-20%) of less than 100° C.
EP-A-0 493 769 discloses terpolymers which are prepared from ethylene, vinyl acetate and vinyl neononanoate or neodecanoate, and their use as additives for mineral oil distillates.
EP-A-0 746 598 discloses copolymers of ethylene and dialkyl fumarates as a mixture with mineral oils which a cloud point of less than −10° C.
The efficacy of the known additives for improving the properties of mineral oil fractions is dependent, inter alia, on the origin of the mineral oil from which they were obtained and, hence, in particular on its composition. Additives which are very suitable for establishing specific properties of fractions of a crude oil can therefore lead to completely unsatisfactory results in the case of distillates of crude oils of different origin.
Against the background of the increased environmental consciousness, fuels which give rise to less environmental pollution during their combustion have recently been produced. Appropriate diesel fuels are distinguished by a very low sulfur content of less than 500 ppm and in particular less than 100 ppm, a low aromatics content and a low density of less than 0.86, in particular less than 0.84, g/ml. They cannot be treated with conventional flow improvers or can be treated therewith only to an inadequate extent. In particular, the winter grades of diesel fuels produced for use under arctic conditions and having extreme low-temperature properties, such as, for example, a cloud point of less than −8° C. and in particular less than −15° C., very narrow distillation cuts with boiling ranges of 20 to 90% by volume below 120° C., in particular below 100° C. and in some cases also below 80° C., and a distillation volume of 95% by volume at temperatures below 360° C., in particular below 350° C. and especially below 330° C., present problems. The low-temperature properties of such distillates can be satisfactorily improved at present only by adding low-boiling, low-paraffin components, such as, for example, kerosene.
The composition caused by narrow distillation cuts and low final boiling points presents problems with regard to the response behavior of flow improvers in these oils. These oils have a paraffin distribution with a maximum at about C12 to C14 and contain only insignificant amounts of the n-paraffins crystallizing out of conventional grades and having hydrocarbon chains longer than C18. The cloud points and CFPP values are so low, especially in the case of winter grades, that conventional flow improvers do not respond and the low-temperature properties must be established by dilution with kerosine.
It was therefore an object of the present invention to develop new fuel oils having an improved low-temperature flowability compared with the prior art.
Surprisingly, it has been found that main chain polymers of ethylene which carry side chains having more than 5 carbon atoms are suitable for lowering the CFPP also in the above described middle distillates. Ethylene/vinyl acetate copolymers having corresponding comonomer contents are on the other hand virtually insoluble in hydrocarbons.